The present invention relates to methods for analyzing earth formations, and more particularly to methods for analyzing subterranean formations comprising alternating thin layers of sand and shale, for determining if producible quantities of hydrocarbons may be present.
In the search for oil and gas deposits, one of the most important and powerful methods for analyzing earth formations penetrated by a borehole is to lower various measuring instruments into the borehole to make various electrical, mechanical, magnetic, nuclear, acoustical, etc., measurements. However, as powerful and sophisticated as these various techniques and tools have become, their resolution is not infinite. In fact, a great deal of effort has understandably been expended to define clearly just what the various tool responses mean. The several companies offering borehole logging services, for example, typically furnish considerable literature, conversion charts, and so forth, to aid in interpreting the tool responses according to the particular formation characteristics at hand.
As an important example having particular reference to the present invention, many hydrocarbon reservoirs having economically important reserves are difficult to evaluate because in their vertical extent they are extremely thin. Even worse, these thin layers may alternate with layers which are not economically producible but produce nearly identical responses on the logging tool. For example, evaluation of thin zones using induction logs is a problem when the zone thickness is less than the vertical resolution of the tool (typically 3 to 6 feet in the case of a deep induction log). The problem is particularly acute when logging in a laminated sand/shale sequence as the log response in such a formation shows essentially no character, and the resistivity of any layer or set of layers is thus not determined. Accordingly, a considerable need exists for a method for accurately analyzing thinly bedded sand/shale formations, and in particular for estimating the average sand resistivity thereof, so that the average fluid content of the sand in such intervals of interest can be determined.